Domestic electrical appliances include both general electrical devices that use a 100 VAC power source, such as lighting fixtures, refrigerators, and TV receivers, as well as appliances that consume more electricity and are designed for a 200 VAC power source, such as air conditioning. A single-phase 3-wire power distribution system that distributes power through common power wires has generally been adopted for these appliances.
As indicated in FIG. 4, this power distribution system uses a pole transformer, which is a power distribution transformer 102, to drop the voltage of a 6600 V high voltage power line 101, and commercial power line, comprising neutral wire N and outer wires P1 and P2, is wired into the building. The neutral wire N is drawn from the middle tap of the secondary coil of the power distribution transformer 102, and outer wires P1 and P2 are drawn from the two edges of the secondary coil. The neutral wire N is generally grounded, and 100 VAC voltage is wired by using the neutral wire N and the outer wire P1 or the neutral wire N and the outer wire P2. In addition, because the potentials of the outer wire P1 and the outer wire P2 have a reverse phase relationship, 200 VAC voltage is distributed between the outer wire P1 and the outer wire P2. Consequently, if the power plug of an appliance is plugged into a power outlet 103 connected to the neutral wire N and the outer wire P1 or to the neutral wire N and the outer wire P2, 100 VAC power is obtained, and if plugged into a power outlet 104 connected to outer wires P1 and P2, then 200 VAC power is obtained.
Meanwhile, there is no particular restriction on whether the power outlet 103 that supplies 100 VAC is connected to the neutral wire N and the outer wire P1 or to the neutral wire N and the outer wire P2, specifically, there is no particular restriction on whether the hot terminal 103p of the power outlet is connected to the outer wire P1 or to the outer wire P2. Moreover, in the past, because 100 VAC power is supplied in either case, there was no problem regarding this connection.
Nonetheless, recently modulation systems such as spread spectrum modulation systems have come into use, and so-called PLC (power line communication) systems that utilize power lines wired into the house to conduct communications have garnered attention; and a practical application of power line communications is to plug the power plugs of modem devices 105 into power outlets 103 that supply 100 VAC and are installed in different locations within the house, and to communicate data between terminal devices connected to the modem devices using the power line as a signal line.
In this kind of power line communications, as indicated in FIG. 4, if the hot terminal 103Ap of the transmitting power outlet 103A and the hot terminal 103Bp of the receiving power outlet 103B are respectively connected to differing outside lines P1 and P2, then the signals that arrive at the receiving terminal will be greatly attenuated, and satisfactory communications will not be maintained. For example, when superimposing a 10 dBm transmission signal on the neutral wire N and outer wire P1 and comparing to the power expressed between the neutral wire N and out-of-phase outer wire P2, in a frequency band of 10 KHz or more there is an attenuation of 30 dBm or more between out-of-phase wiring compared to between in-phase wiring, and demodulation may be become impossible if the signal power becomes −50 dBm.
Thus, in the past, there have been power line circuit selection apparatuses (for example, Japanese Unexamined Patent Application No. H2-50633, which is hereby incorporated by reference in its entirety herein) providing a switching circuit to switch either between the neutral wire N and the outer wire P1 or between the neutral wire N and the outer wire P2, a modem device, and a reception data memory; once the signals flowing between one of the distribution lines were demodulated and memorized in the reception data memory, the data was read from the reception data memory, and newly modulated signals were output to between the other set of distribution lines by using the switching circuit to switch the connection. According to this power line circuit selection apparatus, even if the hot terminals of the power outlets are between terminal devices connected to differing outer wires P1 and P2, the signals are transmitted through the power line circuit selection device, and therefore can be demodulated by the receiving terminal appliance without attenuation.
Nonetheless, the conventional method of connecting this power line circuit selection device between the distribution lines requires the use of a large and expensive device, and is ill-suited for power line communication that conducts communication by using lamp fixture wiring in the house as the signal line while maintaining the previously existing communication environment unchanged as much as possible.
If it is known whether the hot terminal of the power outlet supplying the 100 VAC is connected to the outer wire P1 or P2, then there is no problem, but that connection is not specifically restricted, and the connection status cannot be known from the outer appearance. As a result, when the first power outlet and the second power outlet are connected to differing outer wires, that misconnection cannot be ascertained from the outer appearance, and thus there is the risk that communication continues on without noticing the cause of the attenuation of transmission signal power.
If the hot terminals of a set of power outlets are connected to the same outer wire P1 or P2, the AC voltages supplied between the terminals of the power outlets will be in-phase, and if connected to differing outer wires P1 and P2, the voltages will have reverse phases. Therefore, the aforementioned problem can be resolved if the voltage waveforms of both power outlets are compared, and a set of in-phase power outlets is selected. Nonetheless, in order to simultaneously compare the potential of both, the comparison and determination device must be connected respectively to a set of power outlets, and if the set of power outlets are located separately such as on the first and second floors of the house, then using a long cable to connect the two is not practical, and the connection status cannot be easily determined.